Use of the porous bottom culture dishes (PBCDs) developed in this laboratory for the study of epithelial cells grown in confluent layers continues to grow. This growth represents increased use in more laboratories world-wide and also their application to ever increasing numbers of cell types. This large utilization has resulted in part from the fact that PBCDs are now commercially available from 5 sources (Costar Corp., Funakoski Pharmaceutical Co., ICN Biomedicals, Inc., Millipore Corp., Nuclepore Corp.) with several types of porous membranes, some with cell culture treatments and coatings. The objectives of the research projects being undertaken with these PBCDs go far beyond anything we had in mind at the outset of our development of them. The PBCDs made with collagen membranes also made in this laboratory still provide better optical properties (for phase microscopy) than most of the "homemade" and commercial PBCDs. Many cell types grow well on the collagen also. In fact, we have grown endothelial cells on one side of the collagen membrane and smooth muscle cells on the other side. This makes a good model of a blood vessel. A membrane we are developing to have controlled permeability has even better optical properties than our collagen membranes. This membrane material is reconstituted cellulose to which we give a surface treatment to facilitate cell attachment. The major role played by Ca in the regulation of many cellular processes has stimulated us to try to simplify methods for measuring free Ca activity, so that they will be no more difficult than pH measurements. The use of hydrophobic porous membranes and neutral carrier Ca sensors has resulted in a 2 mm diameter electrode which has rapid response and low resistance. These characteristics allow such electrodes to be used with many pH meters.